Bi-Stable, Sub-Commutated, Direct-Drive, Sinusoidal Motor Controller for Precision Position Control

ABSTRACT

An electric motor controller system for modulating requested motor torque via oscillating the instantaneous torque, including a bi-stable torque controller; a proportional-integral (PI) velocity controller a proportional-integral-differential (PID) position controller; and sinusoidal zero-velocity table mapping.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/US12/50451, filed Aug. 10, 2012, which claims priority to andthe benefit of U.S. Provisional Application No. 61/523,195, filed Aug.12, 2011, both of which are hereby incorporated by reference herein forall purposes.

TECHNICAL FIELD OF ENDEAVOR

The field of the invention relates to the control of direct current (DC)motors, and more particularly to the control of asymmetric direct-drivebrushless DC motors.

BACKGROUND

In order to achieve sub-degree pointing accuracy with brushless DC motorcontrollers, servo drives, stepper motors, and various other motortopologies are chosen for their ease of implementation. Servocontrollers typically require gears to achieve their precision bycommutating through several electrical commutation cycles.Micro-Stepping commutation methods used with stepper motors allow one toincrease their precision beyond the standard capabilities of the systembut are not extendable to direct drive brushless DC motors.

SUMMARY

Embodiments of the invention include an electric motor controller systemfor modulating requested motor torque via oscillating the instantaneoustorque, the system comprising a torque drive oscillating circuitcomprising a sinusoidal drive having at least three phases, where theinstantaneous torque is based on a sinusoidal reference. Additionalembodiments include an electric motor comprising a rotor having three ormore multi-turn coils, each multi-turn coil disposed about an associatedradial arm of the rotor, and a stator having circumferentiallydistributed magnetic elements. Additional embodiments include motortorque feedback and gain compensation, motor velocity feedback and gaincompensation, and angular feedback and gain compensation. In additionalembodiments, the sinusoidal reference may be a sinusoidal zero-velocitytable mapping and may be configured to energize a brushless motor phasethrough a stator of the brushless motor, detect a static condition, andyield a symmetrical three-phase sinusoidal drive table for the brushlessmotor.

Additional embodiments include an electric motor controller forachieving sub-degree pointing accuracy of a brushless direct current(DC) motor comprising a bi-stable torque controller, aproportional-integral (PI) velocity controller, aproportional-integral-differential (PID) position controller, andsinusoidal zero-velocity table mapping. In additional embodiments, thebrushless DC motor may be sub-commutated hundreds of times, i.e.,greater than a hundred times, within one electrical commutation cycle.In additional embodiments, the bi-stable torque controller may beconfigured to oscillate about a request to yield a modulated torquevalue to average a total torque requested of the brushless DC motor. Inadditional embodiments, the bi-stable torque controller may be furtherconfigured to restrict a change in torque to a small fraction of torquechange per second. In additional embodiments, the bi-stable torquecontroller may be further configured to adjust a delta torque morepositive than negative to achieve a gradually modulated torque valuewhen a forward position is requested. In additional embodiments, thebi-stable torque controller may be further configured to draw valuesfrom the sinusoidal zero-velocity table mapping. In additionalembodiments, the proportional-integral (PI) velocity controller may beconfigured to output a result based on a velocity bias and a feedback ofthe brushless DC motor velocity. In additional embodiments, theproportional-integral-differential (PID) position controller may beconfigured to output a result based on a pointing routine and an anglemeasurement feedback. In additional embodiments the sinusoidalzero-velocity table mapping may be configured to advance or retardelectrical degrees to yield a consistent torque curve over all positionswithin the brushless DC motor. In additional embodiments, the sinusoidalzero-velocity table mapping may be implemented in microcode.

Embodiments may also include an unmanned aerial vehicle (UAV) sensorapparatus, comprising a UAV, a direct-drive motor coupled to the UAV,and a sensor coupled to the direct-drive motor, the direct-drive motorconfigured to angularly drive the sensor. In such an embodiment, thedirect-drive motor may be coupled to the UAV through a support.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not limitation in thefigures of the accompanying drawing, and in which:

FIG. 1 depicts an exemplary portion of a brushless DC motor;

FIG. 2 is an exemplary top view of a rotor and a stator of a brushlessDC motor;

FIG. 3 is an exemplary top view of a rotor of a brushless DC motor; and

FIG. 4 depicts a Bi-Stable Motor Control Block.

FIG. 5 is a perspective view of an unmanned aerial vehicle (UAV) with anexemplary sensor gimbal; and

FIG. 6 is a side view of a UAV with an exemplary sensor gimbalapproaching the ground.

DETAILED DESCRIPTION

A technique, utilizing a direct drive brushless DC motor, is implementedby incorporating a bi-stable controller. In most position controlmethods, achieving sub-degree accuracy involves reducing torque ripplewhile minimizing torque changes for a given position. The exemplarymethod involves five components: a direct-drive brushless DC motor, abi-stable torque controller, a proportional-integral (PI) velocitycontroller, a proportional-integral-differential (PID) positioncontroller, and sinusoidal zero-velocity table mapping. The techniquemay be used in a variety of applications, include the control of adirect-drive motor driving a sensor, such as an imager, on an unmannedaerial vehicle (UAV)

Exemplary Brushless Direct Current Motor

FIG. 1 illustrates, in a cut-away view, an exemplary brushless DC motorthat includes an inrunner rotor electrically connected with awye-configuration winding about improved armatures. The DC motor 100 mayhave a casing 102 formed of steel or other high-strength material toenclose and protect the motor 100. A stator 104 is positioned around theperimeter of a rotor 106, with the stator 104 coupled to an iron backing105 to extend the magnetic field of the stator 104. The stator 104 maybe formed of permanent magnets such as neodymium. The rotor 106 may beformed of a ferromagnetic material or a built up laminate. Power andsignal cabling 108 extend through a donut hole 110 in the DC motor 100.An exemplary bi-stable controller may be embodied as a three-phasedriver via embedded microcontroller hardware and circuitry comprisingthree half-bridges.

FIG. 2 is an exemplary top view of a rotor 210 and a stator 212 of abrushless DC motor 200. In this exemplary embodiment, there are morestator poles 208, e.g., permanent magnets, than rotor armatures 204. Therotor 202 in this embodiment has fifteen armatures 204 and the stator212 has sixteen permanent magnets 208. A minimum air gap 206 ismaintained between each armature 204 and its respective magnets 208 asthe armature 204 travels past a magnet 208 during operation. The minimumair gap 206 may be reduced to a distance approaching assembly tolerancesof the motor to prohibit physical contact of the rotor and stator whileincreasing torque of the motor with closer placement of the stator androtor. Opposing walls 210 of adjacent armatures 204 may each be planarand oriented perpendicular to an axis of rotation of the rotor 202. Therotor 202 may have three or more multi-turn coils. Each multi-turn coilmay be disposed about an associated armature 204 of the rotor 202.

FIG. 3 is an exemplary top view of a rotor of a brushless DC motor 300.Each armature 302 may have a coil 304 wrapped around its root section306 to provide its excitation. Each coil 304 may be wrapped havingprogressively more turns as the root section 306 approaches the headportion 308 to enable a greater number of windings per armature 302without impinging on an adjacent winding.

Block Diagram

FIG. 4 depicts a functional block diagram of an exemplary bi-stablemotor controller 400. The block diagram represents the major parts of anexemplary control system, comprising inputs to the system, variousposition and velocity controllers, the bi-stable torque control, andfeedback elements. A requested input angle U₁ (ν_(DESIRED)) is summedwith an offset (θ_(OFF)) to yield a desired mechanical angle (E₁). Thisdesired angle is then conditioned to yield a desired torque which is fedinto the bi-stable torque control. The torque controller may oscillateabout a request to yield a modulated torque value that may move or holdthe motor into the desired position request. The various feedbackelements are filtered and conditioned to reduce noise perturbations andgenerate accurate feedback information to yield a closed loop feedbackcontrol to continue the process. Inputs to the controller are depictedas a pointing routine input 410, an offset control input 411, andfeedback from the angle measurement 412. The input signals are combinedand then amplified or attenuated by a position PID gain 413. The outputof the position PID gain 413 block is combined with a velocity bias 414and feedback of the motor velocity 415, and then amplified or attenuatedby a velocity PI gain 416. The output of the velocity PI gain 416 blockis combined with a torque bias 417 and motor torque 418, amplified orattenuated by a torque gain 419, and the output of the torque gain 419block is depicted as input to the motor 420. Values for the bi-stabletorque gains 419 may be drawn from a sinusoidal lookup 421 that may berepresentative of a sinusoidal zero-velocity table mapping. The outputof the motor 420 is depicted as driving the stabilized platform. Themotor velocity feedback path is depicted as including a motor velocityproportional path, and a discrete derivative path from the output of theencoder angle path. The combination is depicted as filtered and fed intothe controller ahead of the velocity PI gain 416. The position feedbackpath is depicted as including a proportional encoder angle path and adiscrete integrator of the measured motor velocity. The combination isdepicted as filtered and fed into the controller ahead of the positionPID gain 413.

Direct Drive Brushless DC Motor

A direct drive brushless DC controller may reduce the mechanicalcomplexity of the system, thus increasing the reliability and efficiencyof the drive. The brushless DC motor 420 may be resistant to harshcollisions, e.g., the effects of infiltrating dirt and debris, as wellas other environmental factors. Utilizing a direct drive motor 420 forsub-degree accuracy may require sub-commutating the motor 420 hundredsof times within one electrical commutation cycle.

Bi-Stable Torque Controller

By utilizing a current (I) sensor, output torque (τ) can be estimatedthrough the torque constant K_(t) by

${K_{t} = {{2\; {BNlr}} = \frac{\tau}{I}}},$

where N is the number of complete loops of the wire interacting with thepermanent magnetic field, B is the magnetic field strength, l is thelength of the magnet, and r is the radius of the motor armature. Fromthis it can be equated to the velocity constant K_(v) by

${K_{v} = {{\frac{1}{2\; {BNlr}}\mspace{14mu} {and}\mspace{14mu} K_{v}} = {\frac{1}{K_{t},}\mspace{14mu} {thus}}}},{\tau = {\frac{I}{K_{v}}.}}$

The bi-stable torque controller 419 may be implemented via two inputs.Total torque feedback input may be incorporated through a slow varyingfilter measured from the current sensor input. Torque input error may beallowed to change instantaneously depending on the direction of thevelocity controller. Due to the nature of the controller, the stabilizedplatform, as the controlled plant, may oscillate around the desiredstabilization point. The stabilization may be accomplished viacommutating ±X° from the 0° commutation center at a high rate, e.g., at5,000 times per second, and where X may be 83° but may vary from thestandard 60° to 100°. The result of the commanded high rate oscillationis a bi-stable control, where the instantaneous torque oscillates toaverage the total torque requested of the motor.

The oscillatory torque may be stabilized by two exemplaryfunctionalities. A first exemplary functionality comprises therestricting of the change of torque to a small fraction of torque changeper second. The result of oscillating torque very rapidly in bothnegative and positive direction yields the following torque equation.

$\tau_{Tot} = {{{\tau \frac{t_{1}}{T}} - {\tau \frac{t_{2}}{T}\mspace{14mu} {where}\mspace{14mu} T}} = {t_{1} + t_{2}}}$

The first term

${- \tau}\frac{t_{2}}{T}$

is from the positive torque contribution while

$\tau \frac{t_{1}}{T}$

is from the negative torque contribution.

In a static condition there would be an even torque distribution in bothdirections where total torque (τ total) may equal:

${{{\tau \frac{t_{1}}{T}} - {\tau \frac{t_{2}}{T}}} \cong {0\mspace{14mu} {where}\mspace{14mu} T}} = {t_{1} + t_{2}}$

In the case where a forward position is requested, delta torque isadjusted more positive than negative, a t₁ value larger than the t₂value may be produced, and this may result in a gradually modulatedtorque value. Additionally, any imbalance caused by torque ripple maycause small oscillations that may be rectified within the 5 kHz updaterate. With a sinusoidal drive topology, all three phases may be usedsimultaneously to further reduce torque ripple. Torque ripple may beeven further reduced by adding additional phases. For example, a 3-phasesinusoidal drive reduced torque ripple to 5% deviation, 4-phases,5-phases or M-Phases (M>5) may be expected to reduce torque evenfurther.

PI Velocity Controller

For a PI Velocity controller 416 sensor input is differentiated via anencoder at an input frequency of ˜5 kHz, that may be infinite impulseresponse (IIR) filtered with 3 db attenuation with a cut-off frequencyof about ˜1.5 kHz, that outputs to the exemplary bi-stable TorqueController 419.

PID Position Controller

A PID position controller 413 may be embodied as sampling directly froman encoder at an input frequency of @5 kHz 16-Tap finite impulseresponse (FIR) filter with 3 db attenuation with a cut-off frequency ofabout ˜300 Hz.

Sinusoidal Zero-Velocity Mapping

Energizing a motor phase through the stator and waiting for staticconditions results in the alignment of the stator poles to the rotor.The result of the alignment for all electrical phases yields asymmetrical three-phase sinusoidal drive table 421 for the motor. Thistable 421 is then advanced or retarded θ_(AR) electrical degrees toyield a consistent torque curve over all positions within the motor. Acommutation table is a sinusoidal table which repeats the number of polepairs of the motor within the 360° mechanical degrees. The table thentakes the form:

${A*{\sin\left( {{\frac{\theta_{mech}}{\left( \frac{Poles}{2} \right)} \pm \theta_{AR}} + \left( {\left( \frac{2\pi}{3} \right)*P_{x}} \right)} \right)}},{{{where}\mspace{14mu} P_{x}} = {\left\{ {0,1,2,{{\ldots \mspace{14mu} M} - 1}} \right\} \mspace{14mu} {for}\mspace{14mu} M\mspace{14mu} {phases}}},$

θ_(mech) is the input mechanical angle, θ_(AR) is the advance or retardangle, and A is the desired amplitude.

The exemplary equation above, as a sinusoidal reference, may beimplemented directly, i.e., implementing the equation in microcode forexample rather than the look-up table.

FIG. 5 is a perspective view of an unmanned aerial vehicle (UAV) 500with an exemplary sensor gimbal 502. The sensor gimbal 502 may comprisea sensor 504, e.g., an imager, coupled to a direct drive motor (such asthat illustrated in FIGS. 1-3), with the motor in a direct-driveconfiguration with the imager. The motor of the sensor gimbal 502 may becoupled to a fuselage 506 of the UAV 500, such as through a sensorgimbal support 508, to provide an unobstructed view of ground 510 forthe imager. In alternative embodiments, the sensor gimbal 502 may becoupled to a front portion 512 of the fuselage 506 to provide anunobstructed view of both the ground 510 and an airspace in front of theUAV 500, or may be one of a plurality of sensor gimbals extending from abottom surface of port or starboard wings (514, 516).

FIG. 6 is a side view of the UAV 500 with the exemplary sensor gimbal502 approaching the ground 510. In some embodiments, the UAV 500 is aglider or has an electric propulsion system (not shown), and so may havea forward motion 600 during approach and landing with the ground 510.The sensor gimbal 502 is illustrated having a motor 602 to rotate thesensor 504 in a direct-drive configuration on the sensor gimbal 502 to astowed rear-facing angular position 604 for landing. In an alternativeembodiment, the sensor 504 may be rotated to a forward-facing angularposition or may remain in an arbitrary or other pre-determined positionfor landing. As used herein, “direct-drive configuration” means thesensor 504 may be rotatably driven by the motor 602 without the benefitof reduction gears. In one embodiment, the sensor 504 may be coupled toan exterior of the motor 602, with the motor 602 rotating around aninner stator fixed to the gimbal support 508. In an alternativeembodiment, the sensor 504 may be coupled to a rotatable shaft of themotor via a linkage (not shown). In another alternative embodiment, thesensor 504 may be coupled to two or more motors to provide movement ofthe sensor 504 in two or more axes (not shown).

It is contemplated that various combinations and/or sub-combinations ofthe specific features and aspects of the above embodiments may be madeand still fall within the scope of the invention. Accordingly, it shouldbe understood that various features and aspects of the disclosedembodiments may be combined with or substituted for one another in orderto form varying modes of the disclosed invention. Further it is intendedthat the scope of the present invention is herein disclosed by way ofexamples and should not be limited by the particular disclosedembodiments described above.

What is claimed is:
 1. An electric motor controller system formodulating requested motor torque via oscillating the requestedinstantaneous torque, the system comprising: a bi-stable torquecontroller comprising a sinusoidal drive table having at least threephases, wherein the requested instantaneous torque is based on asinusoidal reference.
 2. The electric motor controller system of claim 1further comprising: a brushless electric motor, in communication withthe torque drive oscillating circuit, the brushless electric motorcomprising: a rotor having three or more multi-turn coils, eachmulti-turn coil disposed about an associated armature of the rotor; anda stator having circumferentially distributed magnetic elements.
 3. Theelectric motor controller system of claim 2 further comprising: a motortorque feedback path; a motor velocity feedback path; and an encoderangle feedback path.
 4. The electric motor controller system of claim 3wherein the sinusoidal reference is a sinusoidal zero-velocity tablemapping, and the bi-stable torque controller is configured to draw fromthe sinusoidal zero-velocity table mapping to energize a brushless motorphase through the stator of the brushless motor, detect a staticcondition, and yield a symmetrical three-phase sinusoidal drive tablefor the brushless motor.
 5. An electric motor controller for achievingsub-degree pointing accuracy of a brushless direct current (DC) motorcomprising: a proportional-integral-differential (PID) positioncontroller; a proportional-integral (PI) velocity controller incommunication with the PID position controller; a bi-stable torquecontroller in communication with the PI velocity controller; andsinusoidal zero-velocity table mapping in communication with thebi-stable torque controller.
 6. The electric motor controller of claim 5wherein the brushless DC motor is sub-commutated greater than onehundred times within one electrical commutation cycle.
 7. The electricmotor controller of claim 6 wherein the bi-stable torque controller isconfigured to oscillate about a request to yield a modulated torquevalue to average a total torque requested of the brushless DC motor. 8.The electric motor controller of claim 7 further comprising a currentsensor configured to detect a current through the brushless DC motor. 9.The electric motor controller of claim 8 wherein the bi-stable torquecontroller is configured to receive feedback input through a slowvarying filter measured from an input of the current sensor.
 10. Theelectric motor controller of claim 7 wherein the bi-stable torquecontroller is further configured to restrict a change in torque to asmall fraction of torque change per second.
 11. The electric motorcontroller of claim 7 wherein the bi-stable torque controller is furtherconfigured to adjust a delta torque more positive than negative toachieve a gradually modulated torque value when a forward position isrequested.
 12. The electric motor controller of claim 7 wherein thebi-stable torque controller is further configured to draw values fromthe sinusoidal zero-velocity table mapping.
 13. The electric motorcontroller of claim 12 wherein the sinusoidal zero-velocity tablemapping is configured to use three phases simultaneously to reducetorque ripple.
 14. The electric motor controller of claim 12 wherein thesinusoidal zero-velocity table mapping is configured to use at leastfour phases simultaneously to reduce torque ripple.
 15. The electricmotor controller of claim 6 wherein the proportional-integral (PI)velocity controller is configured to output a result based on a velocitybias and a feedback of the brushless DC motor velocity.
 16. The electricmotor controller of claim 15 wherein the proportional-integral (PI)velocity controller is configured to output the result to the bi-stabletorque controller.
 17. The electric motor controller of claim 15 whereinthe proportional-integral (PI) velocity controller is configured toreceive an input frequency of 5 kHz.
 18. The electric motor controllerof claim 17 wherein the input frequency of 5 kHz is infinite impulseresponse (IIR) filtered with 3 db attenuation and a cut-off frequency of1.5 kHz.
 19. The electric motor controller of claim 6 wherein theproportional-integral-differential (PID) position controller isconfigured to output a result based on a pointing routine and an anglemeasurement feedback.
 20. The electric motor controller of claim 19wherein the proportional-integral-differential (PID) position controlleris configured to sample directly from an encoder.
 21. The electric motorcontroller of claim 20 wherein an input frequency to theproportional-integral-differential (PID) position controller from theencoder is 5 kHz 16-Tap finite impulse response (FIR) filter with 3 dbattenuation and a cut-off frequency of 300 Hz.
 22. The electric motorcontroller of claim 19 wherein the proportional-integral-differential(PID) position controller is configured to output the result to theproportional-integral (PI) velocity controller.
 23. The electric motorcontroller of claim 6 wherein the sinusoidal zero-velocity table mappingis configured to advance or retard 9 degrees to yield a consistenttorque curve over all positions within the brushless DC motor.
 24. Theelectric motor controller of claim 6 wherein the sinusoidalzero-velocity table mapping is implemented via a look-up table.
 25. Theelectric motor controller of claim 6 wherein the sinusoidalzero-velocity table mapping is implemented in microcode.
 26. An unmannedaerial vehicle (UAV) sensor apparatus, comprising: a UAV; a direct-drivemotor coupled to the UAV; and a sensor coupled to the direct-drivemotor, the direct-drive motor configured to angularly drive the sensor.27. The apparatus of claim 28, wherein the direct-drive motor is coupledto the UAV through a support.